Alchemy OS is a MIDlet which creates an operating environment inside a mobile phone. Although it is not an operating system in the full sense, it provides a rich platform to develop, compile, and run applications directly in the phone.

Alpine Linux is a community developed operating system designed for x86 routers, firewalls, VPNs, VoIP boxes, and servers. It has proactive security features like PaX and SSP, which prevent security holes in the software from being exploited. It is based on uClibc and Busybox.

BugOS is a microkernel operating system. It has a kernel, device drivers, a file system, and an Internet module. The main concepts are that every process has its own computer with its own console, security, and modularization. If a process wants to read the file, it asks the kernel. The kernel forwards the request to the filesystem driver, which reads and writes through the partition handler, which operates over the idehdd driver. The kernel is around 20 KB. Processes are fully separated from hardware.

Cadaz GNU/Linux is an operating system designed for PHP developers who always work while mobile. With Cadaz, you can carry not just your work but your entire development environment including OS, complete server stack, and development tools on a flash drive.

Comal-Linux is a Linux distribution derived from Slackware Linux. It is packaged as a live CD, and is intended for desktop users who want to use Slackware Linux without first installing it on their computers. Comal-Linux is built from "pure" Slackware Linux, making it as compatible with the original as possible, including application packages. By choosing lightweight desktop and application software, the distribution can be used on older computers. Comal-Linux is an unofficial Muslim edition of Slackware.

Controlix is a control theory based operating system. It abstracts the complete abstraction hierarchy of computer systems principles, from basic signal-on-wire all the way up to abstract libraries and interface linkage. It is written in the VHDL language and is designed to be modular, synchronous, and retargetable.

Cross Linux From Scratch (CLFS) is a project that
provides you with step-by-step instructions for
building your own customized Linux system entirely
from source. This is a multi-architecture version
of Linux From Scratch that is cross-compiled. It
currently supports Alpha, MIPS, PowerPC, Sparc,
x86, and x86_64.

DragonFly belongs to the same class of operating systems as other BSD-derived systems and Linux. It is based on the same Unix ideals and APIs and shares ancestor code with other BSD operating systems. DragonFly is differentiated from other operating systems in its class by, among others, the HAMMER file system, Virtual Kernels, swapcache, and the pervasive use of soft token locks. DragonFly provides an opportunity for the BSD base to grow in an entirely different direction from the ones taken in the FreeBSD, NetBSD, and OpenBSD series.

GNU Mach is the microkernel upon which a GNU Hurd system is based. It provides an Inter Process Communication (IPC) mechanism which the Hurd uses to define interfaces for implementing in a distributed multi-server fashion the services a traditional operating system kernel provides. Mach is particularly well-suited for SMP and network cluster techniques. Thread support is provided at the kernel level, and the kernel itself takes advantage of that. Network transparency at the IPC level makes resources of the system available across machine boundaries.